Single, dual, and quad-band power amplifier designs typically utilize various forms of ‘emitter follower’ configurations of HBT transistors for power and bias control. Examples of such control circuits may be found in U.S. Pat. No. 6,313,705 and U.S. Pat. No. 5,629,648.
However, emitter follower circuits such as these may demonstrate stability problems, particularly under high power drive. In particular, emitter follower based HBT control circuits are sensitive to oscillations that manifest near peak operating power due to the large changes in the capacitive loading and bias requirements of the power amplifier stage under control. This can cause inductive ringing at the collector of the transistor used for the bias control circuit. Suppressing oscillations is critical for amplifier performance.
An additional, but important aspect of the basic emitter follower type HBT control circuit is the amount of control current necessary to drive the control circuit, which sets the quiescent bias point of the power amplifier. At high operating power levels, RF power is amplified and propagated through a power amplifier (e.g., power amplifier 118 in FIG. 1), and is also incident on the base-emitter junction of a control circuit (e.g., the base-emitter junction of control transistor 108 in FIG. 1). The RF power incident on the emitter of the control circuit causes rectification and increases the amount of control current required for output power control of the power amplifier. Another problem with the basic emitter follower style control circuits is that the sharp power control slope (dB/V) inherent in these circuits can make the power amplifier difficult to control.
Accordingly, it would be helpful to the art of electromagnetic processing to provide more efficient and reliable power amplifier controls.